CNC Router: Mantis Build 2

So here is the next big update to my Mantis build. I left off with the Y axis table done with the sides, front and back all attached to the base. The Y-axis drive nut was installed but the leadscrew and motor were not. So picking up from there I decided to drill out the 5mm holes needed to mount the leadscrews to the motors. I chucked each leadscrew in a lathe and using liberal amounts of oil, I drilled out one end of each leadscrew deep enough to insert the motor shaft fully. As it turns out, I was lucky to get these holes drilled straight and proper in the old lathe I used at work but I did so there are no problems to report. I then used the Loctite 638 to permanently mount the leadscrews to each of the steppers. I then went on to the Z-axis. This is the most complicated assembly in the Mantis since it incorporates the homebuilt spindle assembly into it. I ran into some gotcha’s on this one. As you can see from the image below, the Z-axis is composed of a plate that bolts to the X/Z connector via T-nuts, the 4 bearings, the two rods, two bottom plates and one top plate. You can also see the spindle motor mount and the pins that hold it in place.

That brings me to the first gotcha – David Carr doesn’t mention what he used for pins and how long they are, where he got them, etc… It didn’t take a huge leap of faith to figure out he used steel 1/8th inch diameter dowel pins that were about an inch and a half long. I drilled the two bottom plates and the little motor mount  completely through for these pins. I included these pins in my BOM spreadsheet. I bored a 5/8″ hole through the the two bottom plates for the bronze bushing that will hold the ball bearings for the spindle and hammered the bushing into the hole  – it was a very snug fit so I recommend doing this BEFORE gluing up the entire Z-axis. Then I decided to Loctite the bearings into the bushing so I could size up and cut the spindle shaft (I bought a longer shaft than originally spec’d) and mount it in place before gluing the axis up to prevent knocking it apart later. I did not purchase a bearing sized steel shaft which is like one or two thousands of an inch smaller in diameter to facilitate sliding into ball bearings easier. The use of which explains David’s use of the locking collar. My shaft was tight fitting to the bearing and required careful alignment going in or it would lock up trying to insert it. If I had to do it all over again, I would definitely get the bearing sized shaft. Getting my shaft all the way through both the top and bottom bearings required quite a bit of hammering so it was good that I did this before gluing the axis together. So that was yet another gotcha. Another gotcha I ran into with this assembly is the rod alignment. I matched drilled the top and the two bottom plates to make sure they were parallel but when your trying to glue the rod bushings onto the Z-axis connector plate, you need to make sure that the two bottom plates are completely supported underneath. If you look at the following picture carefully, you can see there is a small gap underneath my bottom plates.

If you then apply pressure onto these plates and the top plate to glue the bearings in place without any support underneath the two bottom plates, you will put a slight bend into the rods and you will get binding and the axis will not move up and down freely. I learned the hard way and had to redo the assembly. I used the steel ruler from a combination square to prop my end blocks up – it just happened to be the perfect thickness to do so. That reminds me of another problem I ran into worth bringing up, I used some old 5 minute epoxy I had sitting around for this assembly and found out that it wouldn’t harden properly and remained rubbery. The rubbery epoxy didn’t adequately hold the bearings in place from the heavy load placed on it from the weight of the axis, the spindle, and gravity. It was a good thing I had to rebuild it to fix the binding issue anyway. So I recommend using brand new epoxy to build the Mantis. Another gotcha is how to make sure the rods are centered and straight on the Z-axis connector plate. I eyeballed the bearings and rods into place and then used a combination square to fine tune the positioning – I used making tape to hold the bearings and rods in place while doing this. I just used the combination square to measure the offset from the left and right edges to the edge of the topmost bearings and then made sure the offset from the edge to the bottom most bearing was the same. This is a pain in the rear but necessary.  Here is a picture of the Z-axis with the X/Z connector plate (plate with the T-nuts).

So once the bearings and rods were re-glued, the axis was very smooth and free moving so it was on to mounting the X-axis rods to the X/Z connector plate. This was pretty easy but the problem here is that you need the match drilled end pieces to guarantee that the rods are parallel and in this case, these are the main Mantis sides and they were screwed to the base already. So I had to unscrew those and set them up on a table to line up the rods. This was very straight forward and I did not have any gotcha’s. One thing you may have noticed from the pictures is that I gave up using screws and washer to hold the rods in place – with all the flipping and flopping and repositioning of the parts while assembling the stages made this a pain so I opted to glue the rods in place. Now it was time to start adding the motors and leadscrews. The mounting of the Z-axis motor is the most complicated just because of it’s position at the back of the X/Z connector plate, I wanted to start with this one but realized that I needed to know if the Z-axis motor and leadscrew would clear the one X-axis  rod it has to go over  so I decided to mount that one first instead. Looking at the picture below, you will see that I did not use a piece of plywood to capture the drive nut for the X-axis like I did for the Y-axis, I found that the was unnecessary and that one screw and some epoxy hold it in place at the edge of the X/Z connector plate just fine. Why make it more complicated then you have to? Space is at a premium on the back of the X/Z connector anyway that any wood you used to capture the drive nut would probably split apart anyway if you tried to screw it in place. So I grabbed a stepper motor with the Z-axis leadscrew Loctited to the shaft and located it into position on the back of the X/Z connector plate with the X-axis rods in place. I then found yet another gotcha! My NEMA17 motors I got from eBay were slightly smaller than David Carr’s even though the faceplate mounting footprint was he same. This put the leadscrew slightly too low and not clearing that one X-axis rod! . Errrrrr! I needed to raise the motor up a bit. I looked at the gap (I didn’t measure it) and thought to myself that a single ply of 1/2″ plywood looked like it would do the trick. I had plenty of scrap 1/2″ ply from the Mantis project so I cut a piece to the right size to fit in between the X-axis rods and then used my band saw to carefully (watch your finger doing this!) slice one ply off. It’s tough to get this just right so it took me a couple of tries. In retrospect, I should have created a guide fence to fit into the bevel guide in the band saw table but I didn’t bother. Here is a picture (a little blurry) of me holding the little Z-axis motor spacer I cut.

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